One of the goals of sustainable development is higher fuel efficiency at lower product cost for systems that use or produce power and heat. This paper presents a second-law-based methodology that aims at this goal. The methodology allows the intensive analysis of design concepts in order to reveal opportunities of fuel and cost savings. The application in this paper is devoted to seawater distillation, The methodology is described, Its application is demonstrated by a design improvement journey for fuel-driven seawater distillation systems. The journey starts with a simple boiler-MSF distiller, reaches a cogeneration configuration and ends with a hybrid configuration worthwhile future development. The journey is presented on a cost-efficiency diagram. Solar-driven distillation is also presented for comparison. Several important issues are discussed.